The present inventon relates generally to an improvement in an end cap for closing the end of a cladding tube of a nuclear fuel element or rod, and more particularly to an end cap provided with a tag gas capsule connected integrally thereto.
During the operation of nuclear reactors there may be a failure of the nuclear fuel element resulting in release of nuclear fuel material and radioactive fission products to the coolant and ultimately to the environment. Moreover, coolant may enter the failed fuel element and react with the fuel materials, causing further damage. It is therefore important to quickly detect the fuel element failure and determine location of the failed fuel element. One of the known methods for detecting a fuel element failure is by analysis of gases which have previously been introduced into the fuel element. This is called "gas tagging", which method makes use of tag gases (also known as "identifying gases"). Various studies have been made on the kinds of gases that are available as the tag gases, gas detecting devices, and so forth. Some reports teach that rare gases such as, for example, xenon, krypton and their isotopes are suitable for introduction into the fuel elements, either alone or in the form of mixtures of predetermined ratio.
In one prior art method for introducing the tag gas into the fuel element, the tag gas is enclosed and sealed in a capsule provided with means for unsealing the capsule, and the thus formed tag gas capsule is placed in the fuel element. When a certain temperature is reached in the fuel element, the unsealing means operates to release the tag gas enclosed in the capsule into the interior of the fuel element. If a failure of the cladding tube of the fuel element occurs in a reactor core, the tag gas escaped from the fuel element makes it possible to determine the location of the failed fuel element by, for example, a mass spectrometric analysis of a sample from the reactor cover gas.
A problem encountered in a practical application of the gas tagging is how the fuel element and the tag gas capsule loaded in the fuel element are associated or corresponded to each other at the time of manufacture of the fuel element. In the typical conventional fuel element, a fuel cladding tube, a bottom end of which is closed by a lower end cap welded thereto, is filled with a plurality of nuclear fuel pellets, and the tag gas capsule is loaded in an upper plenum of the cladding tube, and then an upper end of the cladding tube is hermetically closed by an upper end cap. Thus, in the prior art, the tag gas capsule is constructed as a member which is separate and independent from the end cap. It is therefore necessary to manually record the correspondence between the fuel element and the tag gas capsule each time when the tag gas capsules are loaded into the fuel elements. In addition, there had to be carried out an X-ray non-destructive inspection after the loading of the tag gas capsule, to examine the state of the capsule loaded in the fuel element. Thus, it involves substantial labour in the recording at the time of loading the capsule and in the non-destructive inspection after the loading. Further, due to the limitation on the accuracy of the non-destructive inspection, the reliability of information obtained through the inspection after the loading is also limited practically.